A titled subsection within a chapter or other topic type.
A subsection represents a title component within a publication that contributes to the main table of contents.
Subsections may be nested as needed. As such, they enable creation of arbitrary hierarchies of titled divisions. For example, a chapter may have multiple subsections, whose titles will become wrapped in HTML <h#> elements in an HTML output depending on what level the subsection is. Subsections are useful for organizing content within chapters or articles. Subsections can only contain division, subsection, and sidebar topic types.